Car Starting Voltage Sag

Other Hardware General Electronics
1

Whats the easiest device to handle the sag associated with a car starting? My project is taking 150 mA at 12v. Say the sag time is 5 seconds. I calculated 150000 uF of charge needed (hopefully correct)

Car audio capacitor? Cell phone charger?

Thanks!
Chris

2

If you can't loose power, I'd say use a battery back-up. And if it's critical, how sure are you the car is always going to start in 5 seconds? (Most applications can handle a "reset" during starting.)

A cell phone charger is only going to work if its powered... If the cell phone charger is running off the car battery, it's input is going to sag too...

3

As you have calculated, a capacitor for this is really excessive. Does your project really use 12V? Or does it have a 5V converter somewhere? You need to pick a 5V converter which can work from a lower voltage like 10V. If you look hard enough, it may be possible to find a "low dropout" regulator which will provide stable 5v power from as low as 5.1v input. Of course at that voltage, your car isn't going to start so you have bigger problems.

If you are really using 12V, then what are you using it for? What 12V component is so sensitive that it can't work acceptably well from 11V or lower?

4

wyngnut:
Whats the easiest device to handle the sag associated with a car starting? My project is taking 150 mA at 12v. Say the sag time is 5 seconds. I calculated 150000 uF of charge needed (hopefully correct)

Car audio capacitor? Cell phone charger?

Thanks!
Chris

5s x 0.15A = 0.75C of charge. Capacitor charge for a voltage change dV is CdV, therefore dV = 0.75/C,
a cap of value 0.15F would drop by 5V over that 5 seconds, so not going to really help.

What you need is a boost-buck converter (ie a DC-DC converter with say 5--18V input range and 12V output).

5

Hi,

Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

This way we can offer the best solutions, and all be on the same page.

Thanks... Tom..... :slight_smile:

6

If you have the space, use a small 12volt battery to supply the 150mA and feed this from the car systems via a blocking diode. That way, when the car system droops below 12 volts the small battery feeds your load.